1. Field of the Invention
The present invention relates to an objective optical system of an electronic endoscope.
2. Description of the Prior Art
As conventional objective optical systems of endoscopes, in which (i) a retrofocus-type optical system including a negative first lens group and a positive second lens group has been employed, and (ii) the positive second lens group is arranged to move in the optical axis direction to perform zooming, there have been ones as disclosed in, e.g., Japanese Unexamined Patent Publication (JUPP) No. Sho-51-44937, and JUPP No. Hei-1-279219. However, the objective optical system disclosed in JUPP No. Sho-51-44937 is provided with a small zoom ratio, and with a narrow angle of field of about 90xc2x0 at the short focal length extremity. Furthermore, the objective optical system disclosed in JUPP No. Hei-1-279219 is also provided with a narrow angle of field of about 100xc2x0 at the short focal length extremity.
As examples of objective optical systems of endoscopes provided with a super wide-angle at the short focal length extremity, JUPP No. Hei-8-54561 and JUPP Hei-11-316339 have disclosed objective optical systems having an angle of field of about 130xc2x0 through 140xc2x0 at the short focal length extremity.
In the objective optical system of an endoscope disclosed in JUPP No. Hei-8-54561, a real-image is formed at an intermediate position in the optical system, and zooming is performed by a relay optical system. Consequently, the number of lens elements of the objective optical system is large, and the overall length thereof is long.
In the objective optical system of an endoscope disclosed in JUPP Hei-11-316339, the objective optical system includes the three-lens-group arrangement, i.e., a positive lens group, a negative lens group and a positive lens group; and the second lens group is moved in order to perform zooming. However, the arrangement of the positive first lens group is equivalent to the retrofocus optical system in which the most object-side lens element thereof has negative power for the purpose of attaining a wide angle-of-view. Consequently, the number of lens elements of the objective optical system is large, and the overall length thereof is long. Moreover, zooming is performed by the negative second lens group, so that the diameter of the third lens group becomes larger, if an attempt is made to have a shorter focal length at the short focal length extremity.
The present invention is applied to the objective optical system of an endoscope, in which a negative first lens group and a positive second lens group are provided, and zooming is performed by moving the positive second lens group in the optical axis direction. More specifically, according to the present invention, by setting the lateral magnification within a predetermined range, the objective optical system of an endoscope can enable both observing at a wider angle of field, and enlarged observing at a higher zoom ratio, while the overall length of the objective optical system is maintained shorter, and the diameter thereof is maintained smaller.
According to the present invention, there is provided an objective optical system of an endoscope (hereinafter, the endoscope objective optical system) including a negative first lens group, a positive second lens group, and an imaging device, in this order from the object. A focal length of the entire endoscope objective optical system is changed by moving the positive second lens group in the optical axis direction, and the endoscope objective optical system satisfies the following condition:
m2T less than m2W less than xe2x88x921xe2x80x83xe2x80x83(1) 
wherein
m2T designates the lateral magnification of the second lens group at the long focal length extremity, and
m2W designates the lateral magnification of the second lens group at the short focal length extremity.
The endoscope objective optical system can satisfy the following condition:
xe2x88x921.15 less than f1/fW less than xe2x88x920.5xe2x80x83xe2x80x83(2) 
wherein
f1 designates the focal length of the first lens group, and
fW designates the focal length of the entire endoscope objective optical system at the short focal length extremity.
More concretely, the negative first lens group of the endoscope objective optical system is fixed to the front-end of the endoscope body-insertion portion, and the positive second lens group and the imaging device are supported in the endoscope body-insertion portion in a manner that these are moveable in the optical axis direction. Furthermore, the positive second lens group is moved in order to vary the focal length of the entire endoscope objective optical system, and the imaging device is arranged to move along the optical axis in order to vary the magnification of the endoscope objective optical system, and vary a object distance, which is a distance from the most object-side surface of the objective optical system to an object in an in-focus state.
The negative first lens group can include a negative lens element, or alternatively, includes a negative lens element and a positive lens element, in this order from the object. Specifically, in the case where the negative first lens group includes the two lens elements, the negative first lens group preferably satisfies the following conditions:
n greater than 1.7xe2x80x83xe2x80x83(3) 
3.5 less than f1+/fW less than 25 xe2x80x83xe2x80x83(4) 
wherein
nxe2x80x94 designates the refractive index of the negative lens element in the negative first lens group, and
f1+ designates the focal length of the positive lens element in the negative first lens group.
Furthermore, the endoscope objective optical system preferably satisfies the following conditions:
xe2x88x929.2 less than ODISxe2x80x94w/fw less than xe2x88x924.7xe2x80x83xe2x80x83(5) 
xe2x88x922.2 less than ODISxe2x80x94t/fw less than xe2x88x920.8xe2x80x83xe2x80x83(6) 
wherein
ODIS_w_designates the object distance (a distance from the most object-side surface of the first lens group to an object in an in-focus state) at the short focal length extremity;
ODIS_t designates the object distance at the long focal length extremity; and
fw designates the focal length of the entire endoscope objective optical system.
The negative first lens group can include a negative lens element having at least one aspherical surface. From the aspect of the correcting of aberrations, the aspherical surface is preferably formed so that the lens thickness of the negative lens element having the aspherical surface is lager than that of a negative lens element having a spherical surface with the same paraxial radius of curvature as the paraxial radius of curvature of the aspherical surface at the same height from the optical axis, and the difference between the thickness of the negative lens element having the aspherical surface and the negative lens element having the spherical surface increases as the height from the optical axis increases.
Alternatively, in the case where the negative first lens group includes a positive lens element, the positive lens element can have at least one aspherical surface. The aspherical surface is formed so that the lens thickness of the positive lens element having the aspherical surface is smaller than that of a positive lens element having a spherical surface with the same paraxial radius of curvature as the paraxial radius of curvature of the aspherical surface at the same height from the optical axis, and the difference between the thickness of the positive lens element having the aspherical surface and the positive lens element having the spherical surface increases as the height from the optical axis increases.
As another alternative, in the case where the positive second lens group includes a positive lens element, the positive lens element can have at least one aspherical surface. The aspherical surface is formed so that the lens thickness of the positive lens element having the aspherical surface is lager than that of a positive lens element having a spherical surface with the same paraxial radius of curvature as the paraxial radius of curvature of the aspherical surface at the same height from the optical axis, and the difference between the thickness of the positive lens element having the aspherical surface and the positive lens element having the spherical surface increases as the height from the optical axis increases.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-143531 (filed on May 14, 2001) which is expressly incorporated herein in its entirety.